The invention relates to high-speed data transmission in mobile communications systems, especially when a multichannel configuration is used.
In mobile systems, the transmission capacity available at the radio interface is divided among a plurality of users according to a multiple access principle. The most commonly used multiple access schemes include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA). In TDMA systems, communication over a radio path takes place on a time division basis in successive recurrent TDMA frames, each of which comprises several time slots. A short information packet is transmitted in each time slot in the form of a radio-frequency burst of a limited duration, consisting of a number of modulated bits. The time slots are mainly used for conveying control channels and traffic channels. Traffic channels are used for transmitting speech and data, whereas control channels are used for signalling between a base station and mobile stations. An example of a TDMA radio system is the pan-European mobile system GSM (Global System for Mobile Communications).
In a CDMA system, a traffic channel is determined by a unique spreading code assigned to a mobile station, whereas in an FDMA system a traffic channel is determined by a radio channel.
Maximum data transfer rate on a single traffic channel is limited to a rather low level according to the available bandwidth and the channel coding and error coding used in the transmission. For example, in the GSM system the user data rate of a traffic channel employing one time slot was limited to 9.6 kbit/s according to the original specifications, and the radio interface rate was 12 kbit/s. However, this has been found insufficient for many of the new teleservices, such as telefax, video transmission etc., wherefore new mobile systems are being provided with high-speed data transmission services based on so-called multichannel technology. In multichannel technology, a mobile station is provided with a higher bit rate and a greater bandwidth by means of several parallel basic traffic channels (e.g. several time slots). For example in the GSM mobile system, high-speed data service HSCSD (High Speed Circuit Switched Data) is defined in recommendations GSM 01.34, GSM 02.34 and GSM 03.34 of the ETSI (European Telecommunications Standards Institute). In the HSCSD concept, a high-speed data signal is divided into separate data streams, which are then transferred via N subchannels (N traffic channel time slots) at the radio interface and, correspondingly, via N subchannels between the base station and the mobile services switching centre (transcoder). After the data streams have been divided, they are transferred on the subchannels as if they were mutually independent until they are combined at the receiving end. However, logically these N subchannels are parts of the same HSCSD connection, in other words they form one HSCSD traffic channel. The capacity of an HSCSD traffic channel is thus almost eightfold compared to the capacity of a basic traffic channel, which considerably improves the data transfer rate. The GSM HSCSD is capable of supporting a radio interface rate of 96 kbit/s (8xc3x9712 kbit/s) and user rates of up to 64 kbit/s and 76.8 kbit/s (8xc3x979.6 kbit/s) at the radio interface.
The aim of the EDGE (Enhanced Data Rates for GSM Evolution) project of the ETSI is to develop a new modulation method providing a higher data rate per time slot than the present GMSK modulation, while retaining the channel spacing of 200 kHz and the TDMA frame structure. This enables supporting the present HSCSD data services with a lower number of time slots. The new modulation method also makes it possible to provide new data services with a data rate that may be as high as 64 kbit/s per time slot or over 64 kbit/s (n*64 kbit/s) in a multislot constellation. According to the present alternative modulation methods, the radio interface rate is either 28.8 kbit/s or 38.4 kbit/s on a single channel.
As a result of the new modulation method of the EDGE, the traffic channel data rate at the radio interface and the traffic channel data rate between a base station and an interworking function (usually located remote from the base station at a mobile services switching centre) are no longer directly compatible or adapted one-to-one, unless entirely new rate adaptation functions are defined between the base station and the interworking function.
The EDGE project suggests several new alternative solutions for this problem. One alternative is to define completely new rate adaptation functions, optimized for the EDGE, between the base station and the interworking function. Another alternative manner is to use existing TRAU formats and physical 16 kbit/s channel structures at an Abis interface. An advantage of the latter alternative is that an EDGE radio interface does not require changes in the Abis interface and the TRAUs. Since data rates exceeding 14.4 kbit/s cannot be rate-adapted into one 14.4 kbit/s TRAU frame, TRAU frames of several Abis transmission channels must be used to provide the higher capacity required by the EDGE radio interface. In this case, the base station must process a higher number of Abis transmission channels than the number of time slots used at the EDGE radio interface, which makes the base station more complex. For example, one time slot (channel) at the EDGE radio interface (28.8 kbit/s) would require two Abis transmission channels with a 14.4 kbit/s TRAU format. Correspondingly, two 28.8 kbit/s EDGE channels (57.6 kbit/s multichannel configuration) would require four Abis transmission channels. The EDGE project does not suggest the use of the present 14.4 kbit/s Abis rate adaptations with a 38.4 kbit/s EDGE channel, which results in a more problematic situation. The problem is due to the fact that a 38.4 kbit/s EDGE channel typically requires three Abis transmission channels or 3*14.4=43.2 kbit/s for each EDGE channel, whereupon the data rates at the network interface and at the radio interface are incompatible.
An object of the invention is to simplify the operation and structure of a base station in a mobile system which requires a higher number of transmission channels than traffic channels, and where the total data rates at the radio interface and at the network interface are different.
The invention relates to a method according to claim 1, a mobile system according to claim 10 and a base station according to claim 12.
Each high-speed traffic channel at the radio interface requires two or more lower-rate transmission channels between the base station and the interworking function (network interface), which is typically located at the mobile services switching centre. Radio frames, such as EDGE frames, are transmitted over a radio interface traffic channel. Network interface transmission channels are used for transferring transmission frames, such as TRAU frames. The total data rate at the network interface is higher than the data rate at the radio interface. According to the invention, the total data rates of the network interface and the radio interface are rate-adapted by transferring also fill data in addition to payload in transmission frames between a radio access network element, such as a base station, and the interworking function. In such transmission the average payload data rate at the network interface corresponds to the total data rate at the radio interface, and the combined total data rate of the fill data and the payload corresponds to the total data rate at the network interface. In the uplink direction, the base station inserts the fill data into the transmitted transmission frames and the interworking function removes it as unnecessary from the received transmission frames before the payload is processed further. In the downlink direction, the interworking function inserts the fill data into the transmitted transmission frames and the base station removes it from the received transmission frames before the payload is transmitted in radio frames to the mobile station. In some cases the aforementioned radio access network element may also be some other unit than a base station, for example a radio network controller in a UMTS network.
Due to the invention, for example a 38.4 kbit/s EDGE channel can be adapted in a simple manner to three 14.4 kbit/s Abis channels even though the total data rates are incompatible. Basically the method only comprises manipulating the content of the information to be transmitted, and the method is only visible to the base station and the interworking function which insert and discard the fill data. Therefore the invention does not require any other changes at the radio interface or the network interface, nor does it restrict their further development in any way.
The invention is applicable for both transparent and non-transparent data transmission. However, it is preferable in transparent data transmission that fill data is inserted into the data stream as evenly as possible in order to minimize the variation in transmission delays.